1. Field of Invention
This invention relates to a method of compacting particulate material into dense articles and, more particularly, to a method for the hot isostatic compaction of particulate material into dense articles of intricate configuration.
2. Description of the Prior Art
As is well known, methods of isostatic compaction generally involve placing a mass of particulate material, usually powder, into a container having an internal configuration corresponding to the appropriate precompaction shape of the article to be produced, evacuating and sealing the container and its contents against the atmosphere, placing the container in a pressure vessel wherein isostatic pressure is applied to the container to compact the particulate material into a dense article and thereafter removing the article from the container. Compaction can be conducted at ambient temperatures but generally compaction at elevated temperatures is required to form articles of intricate configuration to high density, especially when the particulate material is a nickel or cobalt-base superalloy powder.
Although the method of isostatic compaction has developed to the degree where articles of high density may be readily obtained, the configuration of such articles has been limited to relatively simple shapes, such as bars, rods or the like, due to the inability of the prior art to devise a suitable container for confining the particles to more complex shapes during compaction. For example, the typical container for compacting powders into articles of simple configuration is one fabricated from metal, such as steel. These so-called metal cans are fabricated to the desired shape by welding sheets or plates of the metal together. However, metal cans of intricate configuration, such as those resembling a gas turbine engine blade, disc and the like, are virtually impossible to construct in this manner. The only practical, existing means by which articles of such configuration can be achieved using metal cans is to subject the compacted article of simple configuration to extensive machining operations. In the case of nickel or cobalt-base superalloys, machining is difficult and time-consuming.
Inherent in the use of metal cans is the further disadvantage that the particulate material may require precompaction to an intermediate density; for example, 70 to 80 percent, prior to final compaction. Precompaction is sometimes necessary because of the inability of the fabricated metal can to shrink to the extent required during compaction of the loose powder (about 50 percent dense) to full density (about 100 percent dense). If the precompaction step is omitted, even an article of simple configuration may exhibit objectionable wrinkles on the surface after compaction.
The inadequacies involved in isostatically compacting with fabricated metal cans resulted in the invention disclosed in U.S. Pat. No. 3,622,313 which issued on Nov. 23, 1971. The method there disclosed comprises sealing a mass of powder in a vitreous container having an internal configuration corresponding to the general shape of the articles to be produced and subjecting the container to hot isostatic compaction. The use of the vitreous container eliminates the need for precompaction of the powder to intermediate density prior to final compaction and enables the production of articles of intricate configuration. However, several disadvantages are associated with the disclosed method. Namely, the vitreous container is fragile and must be handled with care during the operations incident to isostatic compaction. Vacuum integrity of the container is difficult to achieve in thin-walled containers; therefore, thicker walls are necessary and require time-consuming and laborious manufacturing procedures. The surface of the article compacted within the vitreous container is oftentimes rough in nature as a result of the powder sticking to the glass during compaction at high temperatures. Also, the vitreous container tends to sag at elevated temperatures and distortion of the articles being compacted thereby occurs.
Copending U.S. patent application 474,878 now Pat. No. 3,982,934 entitled "Methods Of Powder Metal Formation" filed by Joseph M. Wentzell and assigned to the assignee of the present application discloses a method for isostatically compacting a powdered material, such as superalloy powder, into irregular shapes. Basically, the method comprises forming a thin (2 to 3 mils) electroplated shell in the appropriate precompaction shape of the article to be made, surrounding the shell with a pressure transferring and support media, pressing and sintering the support media, filling the shell with powder to be compacted, placing the filled shell and surrounding support media within a sealable metal can, evacuating and sealing the metal can against the atmosphere, compacting the metal can and powder within a hot pressure vessel wherein isostatic pressure is applied, and removing the metal can, support media and shell from the compacted article. Although the method disclosed is effective in producing compacted articles of intricate and configuration and high density, the steps involved therein are so numerous and timeconsuming as to preclude application of the method in the commercial production of complex articles in large quantities. For example, a pressure transmitting and support media, such as iron powder, is required to surround and support the thin (2 to 3 mils) electroplated shell after the casting has been removed therefrom. The support media must be pressed to a density approximately equivalent to that of the powder to be compacted and thereafter sintered. After the electroplated shell is filled with powder, the filled shell and surrounding sintered support media must then be enclosed within a sealable metal can in order that a vacuum can be maintained in and around the powder during compaction at high temperatures. These steps, as well as the numerous others taught in the application, make the disclosed method impractical from a commercial production standpoint.